Generally, VADs are blood pumps which help the heart pump blood. In certain situations, a patient's heart may need help pumping blood on a short term basis to help the patient recover from a cardiac event or to act as a bridge to heart transplant surgery, or on a long term basis to provide permanent support to the heart. There are several different types of VADs, the most common of which is a left ventricular assist device (LVAD) which helps the left ventricle (LV) of the heart pump oxygenated blood received from the lungs to the rest of the body.
LVADs generally include a pump with an inflow tube connected to the LV of the heart and an outflow tube connected to the aorta. The pump may be a pulsatile flow pump or a continuous flow pump, the latter of which has the advantage of being relatively smaller in size, requires less power, involves fewer moving parts, and is believed to provide some level of auto-control of inflow and outflow pressures.
One type of a continuous flow LVAD is an axial flow pump (AFP) which may use a magnetically suspended rotary impeller (rotor) that is coaxially aligned with the attached ends of the inflow tube and the outflow tube. For example, Thoratec (Pleasanton, Calif.) has developed an AFP type LVAD available under the trade name HeartMate® II. A detailed description of an AFP device is described in U.S. Pat. No. 5,588,812 to Taylor et al.
In operation of such a device, the rotational velocity of the rotor must be sufficient to produce enough blood flow to deliver essential substances to the vital issues and remove products of metabolism, as well as cool the bearings (if used) of the pump and prevent thrombus formation in the pump. However, the rotational velocity of the rotor must not be too high, otherwise a zero or negative pressure may be developed within the left ventricle during diastole, thereby potentially causing ventricular collapse. On occasion if this happens, the LV will collapse momentarily, then refill and collapse again after the LV fills with blood. This oscillatory behavior may be clinically detrimental. Thus, the rotational velocity of the rotor is adjusted to a level that draws the pressure of the LV down to near zero, but not so high as to draw the pressure within the LV to a level at or below zero.
One approach to address this issue is to utilize current from the rotor motor as a feedback mechanism to prevent ventricular collapse. Such an approach is described in U.S. Pat. No. 5,888,242 to Antaki et al. However, it has been shown clinically that using motor current feedback as the only means to prevent ventricular collapse is not always reliable.
Another approach is to provide a pressure transducer in the inflow region of the LVAD and use pressure measurements as a feedback mechanism to infer ventricular collapse. Examples of LVADs incorporating pressure transducers in the inflow tube are described in U.S. Pat. Nos. 6,171,253 and 6,367,333 to Bullister et al., and U.S. Pat. No. 6,481,292 to Reich. However, using inflow pressure to infer collapse is susceptible to error if inflow pressure varies relative to ventricular pressure. In addition, long term use of a conventional pressure sensor is susceptible to pressure drift and attenuation leading to measurement inaccuracies and potentially misleading information transmitted to the LVAD controller, which could lead to either inadequate perfusion (rotational velocity too slow) or ventricular collapse (rotational velocity too fast).
Use of an implanted pressure sensor for feedback is also susceptible to external pressure changes (i.e., changes in barometric pressure). Barometric pressure can change significantly when a weather front moves through the area where the patient resides, when the patient is riding up an elevator in a tall building or traveling in mountainous areas where changes in elevation are frequent and significant. These barometric pressure changes may lead to inaccurate measurements by the implanted pressure transducer, leading to inaccurate feedback data and improper LVAD operation.
Yet another approach is to utilize a sonomicrometer arrangement in combination with a LVAD as described in U.S. Pat. No. 6,540,699 to Smith. A sonomicrometer arrangement utilizes ultrasonic transducers which are allegedly less susceptible to drift than conventional pressure transducers, and therefore may provide a more reliable indication of ventricular collapse. However, in order to obtain an accurate and complete assessment of ventricular collapse, a plurality of ultrasonic transducers should be implanted around the LV. Implanting a plurality of transducers around the heart may be procedurally difficult, and relying on a plurality of transducers increases the likelihood of device failure.
Thus, there is an unmet need for a simple, effective and reliable method of controlling a VAD by drawing the pressure of the LV down to near zero while preventing ventricular collapse.